Electrically switchable liquid crystal-polymer films intended for use in various electro-optical devices have been prepared by mechanical entrapment procedures. One such technique involves imbibing liquid crystal into micropores of a plastic or glass sheet. Another technique involves evaporation of water from an aqueous emulsion of nematic liquid crystal in a solution of water-soluble isotropic polymer such as polyvinyl alcohol.
A different procedure offering significant advantages over mechanical entrapment techniques and the emulsification procedure involves phase separation of liquid crystal from a homogeneous solution with a suitable synthetic resin to form a liquid crystal phase dispersed with an isotropic polymer phase. Techniques of preparing such materials and their advantages are discussed in U.S. Pat. Nos. 4,671,618; 4,673,255; 4,685,771; 4,688,900 and 4,890,902, the disclosures of which are incorporated herein by reference.
The various methods of preparing electrically switchable films as disclosed in the forgoing patents are accompanied by techniques of matching and mismatching the effective index of refraction of the liquid crystal phase with the index of refraction of the polymer to achieve the desired viewing angle in which direct view displays, windows, etc. that incorporate the films may be made viewable or clear. In the case of films made with liquid crystal having positive dielectric anisotropy, the ordinary index of refraction of a liquid crystal typically is matched to the refractive index of the polymer so that in a field-ON state, the display or window appears visible or clear because the optical axes of refraction of the liquid crystal are aligned parallel to the field and normal to the viewing surface. In the field-OFF state, the optical axes of the liquid crystal and polymer are misaligned and randomly oriented so that incident light is scattered and the display or window appears opaque.
In windows or other direct view displays as described above in which the ordinary index of refraction of the liquid crystal is matched to the refractive index of the polymer, the device appears most transparent (field-ON state) when viewed along the direction of the field, which is usually normal to the viewing surface. Transparency decreases giving rise to increasing "haze" at increasing oblique viewing angles until an essentially opaque appearance is detected at an oblique enough angle. This condition of haze results from the fact that the farther the viewing angle is from the orthogonal, the greater is the mismatch between the effective index of refraction of the liquid crystal and the refractive index of the matrix.